A variety of commercial processes rely on fluid separation techniques using membranes in order to separate one or more desirable or undesirable fluid components from a mixture. Separation processes using membranes are used for the separation of water from mixtures with organic liquids, for the separation of volatile organic compounds from aqueous solutions, for the separation of organic components from mixtures of same, or for the separation of at least one volatile component from a mixture with at least one nonvolatile component.
This type of membrane separation operates on the basis of differences in permeation rate through certain dense, non-porous membranes. When the mixture to be separated is brought into contact with the membrane as a liquid, the process is called pervaporation. If the mixture is gaseous, the term "vapor permeation" is often applied. The present invention applies to both processes, but in the present specification, for the sake of brevity, the word pervaporation will be used to represent both processes. In both cases, one side of the membrane faces the fluid mixture while the other side is exposed to a vacuum or a carrier gas, which reduces the partial pressure of the permeable substance and thereby provides the driving force for permeation.
In passing through the membrane, a substance is first sorbed or absorbed into the membrane, then it diffuses through the membrane, and finally emerges as a gas on the low pressure side of the membrane. Different substances will permeate at different rates according to the chemistry of the membrane material and the prevailing operating conditions interacting with it. Some membranes favor the permeation of water over organic substances: these are termed "hydrophilic". Those favoring organics over water are termed "hydrophobic". Other membranes are designed to separate different species of organic substances.
The components of the fluid that pass through the membrane comprise the "permeate" and those that do not pass comprise the "retentate." The valuable fraction from the process may be the retentate or the permeate or in some cases both may be valuable.
Even mixtures such as azeotropes can be effectively separated by pervaporation, which is not possible utilizing thermodynamic vapor-liquid equilibria, such as in distillation processes. Numerous mixtures, e.g. water and ethanol, water and isopropanol, chloroform and hexane, water and tetrahydrofuran, water and dioxane, methanol and acetone, methanol and benzene, methanol and methylacetate, ethanol and ethylacetate, ethanol and cyclohexane, and butanol and heptane, which vaporize azeotropically when certain concentration limits are reached, can be separated by pervaporation.
U.S. Pat. No. 5,536,405 which issued Jul. 16, 1996 to Myrna et al. discloses a stacked membrane disk assembly which is located in a pressure vessel. This is typical of many commercial apparatus, which require pressure vessels to operate. Additionally, many pervaporation processes are operated at elevated temperatures, e.g. 100.degree. C. Apart from the capital expense of pressure vessels, one of the disadvantages of having a pressure vessel is that the vessel needs to be dismantled when repairs are required to be performed on the membrane disk assembly which is inside the vessel. The down-time for dismantling, replacing disks or the disk assembly and then reassembling the apparatus can be as long a day or more.
U.S. Pat. No. 5,620,605 which issued Apr. 15, 1997 to Jens K. Moller discloses an apparatus having membrane cassettes which may be operated with vacuum on the permeate side of the membrane and atmospheric pressure on the retentate side of the membrane. However, this is a huge and complex apparatus that would be very difficult and time consuming to repair should some of the membrane cassettes fail.